The superconducting properties, microstructures and chemical compositions of sheathed MgB2 tapes, and of one wire, synthesized by using the powder-in-tube method, were investigated. At 4.2K, critical current densities of up to 105A/cm2 (0T) and 1.5 x 104A/cm2 (2.5T) were obtained by transport measurements in the wire and the tapes, respectively. In the MgB2 matrix of all of the samples, O was identified and mole fractions of 0 to 10at% were determined by means of electron probe micro-analysis. It was found, by scanning electron microscopy, that only the tapes exhibited B-rich secondary phases; about 10µm in size. A comparison of various tapes showed that the critical current increased with the aspect ratio, and decreased with the O mole fraction in the MgB2. In the tapes, the aspect ratios were inhomogeneous and the critical current densities at low fields were limited by insufficient thermal stabilization. In order to understand the internal oxidation of the MgB2 tapes, one tape was investigated by using analytical transmission electron microscopy. A combination of energy-dispersive X-ray spectroscopy and electron spectroscopic imaging by transmission electron microscopy yielded phase maps for this sample. This revealed an heterogeneous microstructure, with MgB2 grain sizes of between 20nm and 1µm. The O was primarily bound in 20nm to 1µm MgO precipitates and secondary phases, and no boron oxides could be detected. Randomly distributed, 50nm to 1µm B-rich secondary phases (MgB4+δ, MgB7+γ), embedded in the MgB2 matrix were identified.

Superconducting Properties, Microstructure and Chemical Composition of MgB2 Sheathed Materials. D.Eyidi, O.Eibl, T.Wenzel, K.G.Nickel, S.I.Schlachter, W.Goldacker: Superconductor Science and Technology, 2003, 16, 778-88